Magnetic stimulus of isfet-based sensor to enable trimming and self-compensation of sensor measurement errors

ABSTRACT

An ion sensor apparatus comprises at least one ion sensitive field effect transistor (ISFET) device configured to be exposed to a liquid, a reference electrode configured to contact the liquid to which the ISFET device is exposed, and at least one magnet configured to intermittently expose the ISFET device to a magnetic field. A processor is operatively connected to the ISFET device and the reference electrode. The processor modulates the magnetic field to produce a corresponding modulated output in resistance of the ISFET device, and modulation of a reported output value of the ion sensor apparatus.

BACKGROUND

An Ion Sensitive Field Effect Transistor (ISFET) device is typicallyimplemented in a pH sensor used for measuring ion concentrations insolution. When the ion concentration (such as H⁺) in the solutionchanges, the current through the ISFET device will change accordingly.The solution is used as the gate electrode of the transistor formed bythe ISFET device.

In the environment of deep sea pH sensing, large cyclic pressure changesover depth can lead to deformation and creep of pH sensor packaging thatincludes an ISFET sensor die. This mechanical change of the pH sensorpackaging can alter the stresses on the sensor die and changes itsresistance due to piezoresistance effects. These changes are not readilypredictable and cannot be compensated for in the initial calibration ofthe pH sensor in the factory. In addition, changes in the pH sensor aretypically not directly observable over time, which can lead to poorperformance over the lifetime of the pH sensor.

SUMMARY

An ion sensor apparatus comprises at least one ion sensitive fieldeffect transistor (ISFET) device configured to be exposed to a liquid, areference electrode configured to contact the liquid to which the ISFETdevice is exposed, and at least one magnet configured to intermittentlyexpose the ISFET device to a magnetic field. A processor is operativelyconnected to the ISFET device and the reference electrode. The processormodulates the magnetic field to produce a corresponding modulated outputin resistance of the ISFET device, and modulation of a reported outputvalue of the ion sensor apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

Features of the present invention will become apparent to those skilledin the art from the following description with reference to thedrawings. Understanding that the drawings depict only typicalembodiments and are not therefore to be considered limiting in scope,the invention will be described with additional specificity and detailthrough the use of the accompanying drawings, in which:

FIG. 1 is a schematic diagram of an ion sensor apparatus according toone embodiment;

FIG. 2 is a schematic diagram of an ion sensor apparatus according toanother embodiment;

FIG. 3 is a schematic diagram of an ion sensor apparatus according to afurther embodiment;

FIG. 4A is a schematic perspective view of a semiconductor device thatcan be implemented as an Ion Sensitive Field Effect Transistor (ISFET)die according to one embodiment;

FIG. 4B is a schematic top view of the semiconductor device of FIG. 4A;

FIG. 5 is a schematic perspective view of a semiconductor device thatcan be implemented in an ion sensor apparatus according to oneembodiment; and

FIG. 6 is a schematic perspective view of a semiconductor device thatcan be implemented in an ion sensor apparatus according to anotherembodiment.

DETAILED DESCRIPTION

In the following detailed description, embodiments are described insufficient detail to enable those skilled in the art to practice theinvention. It is to be understood that other embodiments may be utilizedwithout departing from the scope of the invention. The followingdetailed description is, therefore, not to be taken in a limiting sense.

An ion sensor apparatus is provided that utilizes Ion Sensitive FieldEffect Transistor (ISFET) technology along with a magnetic stimulus toenable trimming and self-compensation of sensor measurement errors. Inone embodiment, the ion sensor apparatus can be implemented as a pHsensor for liquids.

The ion sensor apparatus generally includes at least one packaged ISFETdie, a reference electrode, an optional counter electrode, and avoltage/current supply to provide power to the ISFET die. A processorconnects the ISFET die and the electrodes in order to make a pHmeasurement. The ion sensor apparatus further includes a magnet forapplying the magnetic stimulus to the ISFET die. Error compensationsoftware is implemented by the processor to apply compensation forsensor errors based on the magnetic stimulus.

In one embodiment, the magnet is an electromagnet that has at least onecoil of wire. A power supply can be provided to drive the coil of wireto create an electromagnetic field to which the ISFET die is exposed.

In another embodiment, one or more permanent magnets are employed togenerate the magnetic field. To produce a “chopper” effect of being ableto turn off and on the magnetic field, and measure the ISFET dieresponse, the permanent magnet can be moved closer or further away fromthe ISFET die, or a magnetic shield can be periodically placed betweenthe magnet and the ISFET die to alter the magnitude of the magneticfield. While permanent magnets are less controllable, they do notconsume electrical energy, which is beneficial for applications withlimited power available.

In addition, a sealed magnetic field sensor (e.g., not exposed to liquidor pressure) can be employed to provide feedback on the magnitude/stateof the magnetic field. With application of the magnetic stimulus,essentially constant levels of pH can be measured, and corruptinginfluences like temperature, pressure, and mechanical changes on theISFET die can be trimmed out.

To one skilled in the art of semiconductors it is known that thevelocity by which electrons and holes move through a semiconductor isproportional to the carrier mobility of the semiconductor material.Different semiconductors have different inherent levels of mobility. Itis also known that the electrical conductivity/resistivity of asemiconductor is proportional to the semiconductor's mobility. Changingmechanical stresses and temperature can impact the mobility of carriersleading to changes to resistance, which leads to changes in measured pH.

It is also known that magnetic fields can influence the flow of carriersthrough a material and in particular semiconductors. This effect isexploited to create Hall effect sensors. The Hall effect relies on thephenomenon that given a flow of carriers along one axis and a magneticfield applied about an axis perpendicular to the first axis, thecarriers will start flowing towards a third perpendicular axis. Thisdistortion of the carrier flow results in a Hall potential voltage thatcan be measured along this third axis. In the axis in which the carrierswere originally and predominately flowing, the distortion of the carrierflow gives rise to an apparent increase in resistance along this axis.

The present technique is not concerned about the Hall potential that isgenerated in the off axis, but in the change in resistance that occursas carriers flow through the ISFET device from source to drain throughthe gate connection, that is, their normal direction of flow. A changein voltage can be measured due to the change in resistance from sourceto drain in the ISFET device.

In the present approach, a modulated magnetic field is applied toproduce a corresponding modulated output in resistance of the ISFETdevice and modulation of the reported pH value. The applied magneticfield can be used as a “chopper” signal to provide a known inputexcitation that should produce an expected output. When there is adifference between the actual output and the expected output,compensation can be applied to null this difference over time, therebymaintaining sensor stability.

One feature of the magnetic stimulus is that the signal it generates inthe ISFET die is passed onto signal conditioning components (e.g.,microprocessor, analog to digital converters, amplifiers, etc.). Inaddition, changes to the performance of these components with time dueto aging are rolled into the error in pH sensing at the ISFET channel.Therefore, the present approach can also be used to trim these otherdrift terms that are normally not correctable.

In one implementation, the ion sensor apparatus can be employed as a pHsensor for deep sea/oceanographic instrumentation research. In thisimplementation, the pH sensor can be made robust enough to work atdepths of about 6000 m for up to about 5 years, while maintainingexceptional stability and sensor accuracy.

Further details of the present ion sensor apparatus are describedhereafter with reference to the drawings.

FIG. 1 illustrates a sensor apparatus 100 for sensing ions in a liquidaccording to one embodiment. The sensor apparatus 100 generally includesat least one ISFET device 110 configured to be exposed to the liquid, areference electrode 112 configured to contact the liquid, at least onemagnet 120, and a processor 130 operatively connected to ISFET device110 and reference electrode 112. The magnet 120 is configured tointermittently expose ISFET device 110 to a magnetic field.

In one embodiment, ISFET device 110 is disposed within a flow tube 140that is configured to receive flowing liquid designated by arrows 150.The reference electrode 112 is also in communication with the liquidinside flow tube 140.

The processor 130 modulates the magnetic field to produce acorresponding modulated output in resistance of ISFET device 110 andmodulation of a reported output value of sensor apparatus 100.

In one exemplary implementation, ion sensor apparatus 100 is configuredto be submersed in sea water, such as for deep sea pH sensing. As such,the various components of ion sensor apparatus 100 can be packaged in awater tight housing structure 150 that is able to withstand deep seaconditions. Other exemplary implementations for sensor apparatus 100include ion sensing of ground water, industrial chemicals, refinedchemicals, crude oil, or the like.

In a method of compensating for ion sensor measurement errors in sensorapparatus 100, ISFET device 110, while in contact with a liquid, isexposed to a magnetic field from magnet 120. This produces acorresponding output in resistance of ISFET device 110. This actualoutput of ISFET device 110 is monitored by processor 130 to determinewhether there is a difference between the actual output and an expectedoutput based on the magnetic field, and an error compensation value isdetermined when a difference in the outputs is detected. The errorcompensation value is then applied to null the difference between theactual output and the expected output over a period of time to maintainstability of the ion sensor measurements.

FIG. 2 illustrates an ion sensor apparatus 200 according to anotherembodiment. The sensor apparatus 200 generally includes an ISFET die 210configured to be exposed to a liquid, an electromagnet 220 configured toexpose ISFET die 210 to a magnetic field, and a processor unit 230operatively connected to ISFET die 210 and electromagnet 220.

The ISFET die 210 includes a semiconductor substrate 211 such as asilicon substrate. A source 212, a drain 214, and an ion sensitive gatechannel 216 are formed in semiconductor substrate 211 with conventionaltechniques. The source 212 is electrically connected to a voltagereference source 218, which is coupled to processor unit 230. The drain214 is electrically connected to processor unit 230. The ISFET die 210is configured such that during operation of sensor apparatus 200, anominal carrier direction is in a first direction, a magnetic field isin a second direction perpendicular to the first direction, and a Halleffect is in a third direction perpendicular to the first and seconddirections, as shown in FIG. 2.

In another embodiment, the drain and source connections on the ISFET diecan be reversed with a switch, so that the nominal direction thecarriers flow can be reversed. Combining this with different strategiesfor changing the sign of the magnetic field vector can be used to cancelout biases in the sensor output. Some errors, such as Ettinghausen,Peltier, and Joule heating errors, can be reduced by this method.

The electromagnet 220 includes at least one wire coil 222 that generatesan electromagnetic field when power is applied. In one embodiment, acoil of wire can be placed around ISFET die 210 to produce theelectromagnet field. A stable AC or DC power supply can be provided todrive wire coil 222, and a precision ammeter or voltmeter can beimplemented to measure the current or voltage across wire coil 222. Inone embodiment, a voltage control signal line 224 connects processorunit 230 to a voltmeter 226 coupled to wire coil 222. The electromagnet220 is selectively activated by processor unit 230 to intermittentlyexpose ISFET die 210 to the electromagnetic field produced byelectromagnet 216.

In another embodiment, electromagnet 220 can include two or more coils.For example, two concentric and parallel offset coils can be used tocreate a uniform and intense magnetic field. Pairs of these coilsaligned to orthogonal axes can create a multi-axis Helmholtz coil thatallows magnetic fields to be generated with an arbitrary magnetic vectororientation in space. This allows for controlling the direction in whichthe carriers in ISFET die 210 are distorted and the impact this has onresistance.

A feature of an AC generated magnetic field is that the stimulusgenerated in the output of the ISFET is also AC. High frequency signalswill be less impacted by flicker or 1/f noise that occurs in thesecircuits, thereby giving a higher signal to noise ratio. The frequencyapproach also lends itself to demodulation techniques that provide goodsignal to noise vs DC measurements.

The processor unit 230 includes various components, such as signalconditioners, amplifiers, analog to digital (A/D) converters,microprocessor algorithms, and the like, for processing the signalsreceived from ISFET 210 in order to produce a pH measurement.

The sensor apparatus 200 further includes a reference electrode 232 anda counter electrode 234, which are configured to contact the liquid towhich ISFET die 210 is exposed. Both of reference electrode 232 andcounter electrode 234 are electrically connected to processor unit 230.The counter electrode 234 provides for grounding out parasitic effectsand filters out noise that can interfere with sensor measurements.

A sealed magnetic field sensor 236, such as a Hall sensor or other typeof magnetometer, can be located adjacent to electromagnet 220 andelectrically connected to processor unit 230. The magnetic field sensor236 provides feedback on the magnitude or state of the electromagneticfield produced by electromagnet 220 to processor unit 230.

A pump can be employed to flow a liquid across ISFET die 210 duringoperation. The ISFET die 210 can be located within a flow tube that isconfigured to receive flowing liquid designated by arrows 240.

In one exemplary implementation, ion sensor apparatus 200 is configuredto be submersed in sea water, such as for deep sea pH sensing. As such,the various components of ion sensor apparatus 200 can be packaged in asealed housing structure that is able to withstand deep sea pressure andtemperature conditions. Other exemplary implementations for sensorapparatus 200 include ion sensing of ground water, industrial chemicals,refined chemicals, crude oil, and the like.

In a method of compensating for ion sensor measurement errors in sensorapparatus 200, ISFET die 210, while in contact with a liquid such as seawater, is exposed to a magnetic field from electromagnet 220. Thisproduces a corresponding output in resistance of ISFET die 210, which ismonitored by processor unit 230 to determine whether there is adifference between this actual output and an expected output based onthe magnetic field, and an error compensation value is determined when adifference in the outputs is detected. The error compensation value isthen applied to null the difference between the actual output and theexpected output.

FIG. 3 illustrates an ion sensor apparatus 300 according to anotherembodiment. The sensor apparatus 300 generally includes an ISFET die 310configured to be exposed to a liquid, at least one permanent magnet 320configured to expose ISFET die 310 to a magnetic field, and a processorunit 330 operatively connected to ISFET die 310.

The ISFET die 310 includes a semiconductor substrate 311 such as asilicon substrate. A source 312, a drain 314, and an ion sensitive gatechannel 316 are formed in semiconductor substrate 311 with conventionaltechniques. The source 312 is electrically connected to a voltagereference source 318, which is coupled to processor unit 330. The drain314 is also electrically connected to processor unit 330. The ISFET die310 is located within a flow tube 340 that is configured to receiveflowing liquid designated by arrows 350.

The permanent magnet 320 is located outside of flow tube 340 butpositioned such that ISFET die 310 can be exposed to the magnetic fieldof permanent magnet 320. A magnetic shield 342 surrounds the outside offlow tube 340 where ISFET die 310 is located. The magnetic shield 342 isconfigured to intermittently expose ISFET die 310 to the magnetic fieldof permanent magnet 320.

In one embodiment, the magnetic shield 342 has a disc-shaped structurewith holes 344 for exposure of ISFET die 310 to the magnetic field ofpermanent magnet 320. A rotary actuator 346 can be coupled to magneticshield 342 to provide rotation of magnetic shield 342, from a closedposition that blocks the magnetic field, to an open position that allowsISFET die 310 to be exposed to the magnetic field of permanent magnet320 when holes 344 are aligned with the north and south magnetic poles(N and S). The operation of rotary actuator 346 can be controlled byprocessor unit 330 with conventional techniques.

In another embodiment, a pair of permanent magnets can be implemented insensor apparatus 300, such that ISFET die 310 is exposed to oppositepoles of each magnet. For example, ISFET die 310 can be exposed to themagnetic field produced between the N pole of one magnet and the S poleof the other magnet.

The processor unit 330 includes various components, such as signalconditioners, amplifiers, A/D converters, algorithms, and the like, forprocessing the signals received from ISFET die 310 in order to produce apH measurement.

The sensor apparatus 300 further includes a reference electrode 332 anda counter electrode 334, which are in communication with the liquid inflow tube 340. Both reference electrode 332 and counter electrode 334are electrically connected to processor unit 330 through an A/Dconverter.

A magnetic field sensor 336 is located in line with the magnetic fieldof permanent magnet 320, and is electrically connected to processor unit330. The magnetic field sensor 336 provides feedback on the magnitude orstate of the magnetic field of permanent magnet 320 to processor unit330.

In regard to flowing liquid over ISFET die 310, it may be desired thatthe flow stop during the magnetic stimulus. For example, the combinationof flowing electrically conductive water (e.g., sea water) and amagnetic field may generate a current flow or potential on nearbyelectrodes or ISFET die 310 due to magnetohydrodynamics. Thus, a flowcontrol valve 348 can be positioned near an opening of flow tube 340 tostop the flow of water during the magnetic stimulus of ISFET die 310.The flow control valve 348 can be operated by a controller 349 to openand close to the valve as needed.

The sensor apparatus 300 can also include at least one temperaturesensor 352, and a pressure sensor 354, which are in communication withthe liquid in flow tube 340. Both of temperature sensor 352 and pressuresensor 354 are electrically connected to processor unit 330 through anA/D converter. In one embodiment, two or more temperature sensors can bearranged spatially around ISFET die 310 to allow for the determinationof thermal gradients, which can be used for error compensation.

In one implementation, ion sensor apparatus 300 is configured to besubmersed in sea water, such as for deep sea pH sensing. As such, thevarious components of ion sensor apparatus 300 can be packaged in asealed housing structure that is able to withstand deep sea pressure andtemperature conditions. Other exemplary implementations for sensorapparatus 300 include ion sensing of ground water, industrial chemicals,refined chemicals, crude oil, and the like.

In a method of compensating for ion sensor measurement errors in sensorapparatus 300, ISFET die 310, while in contact with a liquid such as seawater, is exposed to a magnetic field from permanent magnet 320. Thisproduces a corresponding output in resistance of ISFET die 310, which ismonitored by processor unit 330 to determine whether there is adifference between this actual output and an expected output based onthe magnetic field, and an error compensation value is determined when adifference in the outputs is detected. The error compensation value isthen applied to null the difference between the actual output and theexpected output.

FIGS. 4A and 4B illustrate a semiconductor device 400 according to analternative embodiment, which can be implemented as an ISFET die in thepresent ion sensor apparatus. The device 400 includes a semiconductorsubstrate 404 such as a silicon substrate, which has a top surface 408.A source 412 and a drain 414 are located in opposing side portions ofsubstrate 404 below top surface 408. A pair of separate electrical Hallcontacts 420 and 422 are located in opposing side portions of substrate404 below top surface 408 and adjacent to the side portions where source412 and drain 414 are located. A gate channel 426, which is sensitive toions in a liquid, is located in a top central portion of substrate 404between source 412 and drain 414, and between Hall contacts 420, 422.The device 400 can be fabricated using conventional semiconductorprocessing techniques.

The source 412, drain 414, and Hall contacts 420, 422 are separate fromeach other, and located below top surface 408 of substrate 411 to avoidcontact with a liquid media during operation of device 410 in an ionsensor apparatus. The device 410 allows for measurement of two voltages,with one voltage measurement from Hall contacts 420, 422, and the othervoltage measurement from source 412 and drain 414.

FIG. 5 illustrates a semiconductor device 500 according to analternative embodiment, which can be implemented in the present ionsensor apparatus. The device 500 includes an ion sensitive die 502comprising a semiconductor substrate 504 such as a silicon substrate,which has a top surface 506. A plurality of contacts, such as contacts508 a-508 d, are located in opposing side portions of substrate 504below top surface 506. An ion sensitive channel 510 is located in a topcentral portion of substrate 504 between the contacts. The contacts areseparate from each other, and located below top surface 506 of substrate504 to avoid touching a liquid during operation of device 500 in an ionsensor apparatus. Although substrate 504 is depicted as having acircular shape, substrate 504 can be fabricated to have other geometricshapes. The die 502 can be fabricated using conventional semiconductorprocessing techniques.

Each of contacts 508 a-508 d is coupled to respective contact pads 512a-512 d, which are part of a rotary switch 514. Each of contact pads 512a-512 d is switchably connectable to a voltage reference source (Vref)516 or to a processor unit 520, which can be configured for ISFET pHprocessing.

During operation, different contacts of ion sensitive die 502 can beselectively connected through rotary switch 514 to act as a source anddrain. For example, when Vref 516 is coupled to pad 512 a through rotaryswitch 514, contact 508 a is electrically connected to Vref 516 and actsas a source. Also, when processor unit 520 is coupled to pad 512 cthrough rotary switch 514, contact 508 c is electrically connected toprocessor unit 520 and acts as a drain. In another configuration, whenVref 516 is coupled to pad 512 b through rotary switch 514, contact 508b is electrically connected to Vref 516 and acts as a source. In thisconfiguration, when processor unit 520 is coupled to pad 512 d throughrotary switch 514, contact 508 d is electrically connected to processorunit 520 and acts as a drain.

FIG. 6 illustrates a semiconductor device 600 according to anotheralternative embodiment, which can be implemented in the present ionsensor apparatus. The device 600 includes an ion sensitive die 602 thathas similar features as die 502 (FIG. 5). Accordingly, die 602 includesa semiconductor substrate 604 that has a top surface 606. A plurality ofcontacts, such as contacts 608 a-608 d, are located in opposing sideportions of substrate 604 below top surface 606. An ion sensitivechannel 610 is located in a top central portion of substrate 604 betweenthe contacts. The contacts are separate from each other, and locatedbelow top surface 606 of substrate 604 to avoid touching a liquid beingtested.

Each of contacts 608 a-608 d is coupled to respective contact pads 612a-612 d, which are part of a rotary switch 614. Each of contact pads 612a-612 d is switchably connectable to a Vref 616 or to a processor unit620, which can be configured for ISFET pH processing. In addition, eachof contacts 608 a-608 d is coupled to respective contact pads 622 a-622d that are part of a rotary switch 624, which is configured for Halleffect potential measurements.

During operation of device 600, opposite orthogonal contacts areconnected when making pH measurements and/or Hall potentialmeasurements. Accordingly, different contacts of ion sensitive die 602can be selectively connected through rotary switch 614 to act as asource and drain. For example, when Vref 616 is coupled to pad 612 athrough rotary switch 614, contact 608 a is electrically connected toVref 616 and acts as a source. Also, when processor unit 620 is coupledto pad 612 c through rotary switch 614, contact 608 c is electricallyconnected to processor unit 620 and acts as a drain.

In addition, those contacts of die 602 not used as the source and draincan be selectively connected to a voltmeter by rotary switch 624 toallow for Hall effect potential measurements. For example, when thevoltmeter is coupled to pad 622 b through rotary switch 624, contact 608b is electrically connected to the voltmeter and acts as a Hall contact.Likewise, when the voltmeter is coupled to pad 622 d through rotaryswitch 624, contact 608 d is electrically connected to the voltmeter andalso acts as a Hall contact.

Although the dies in the embodiments of FIGS. 5 and 6 are depicted withfour contacts, more contacts can be added to the dies so long asopposite orthogonal contacts are connected when making pH measurementsand/or Hall potential measurements.

The dies of FIGS. 5 and 6 can provide various error mitigating benefitswhen employed. For example, the dies can mitigate errors introduced by amaterial inhomogeneity in the dies. In addition, the dies can mitigateerrors introduced by unintended misalignment between a silicon wafer andsemiconductor processing masks, as well as depositions, when patterningthe die features during fabrication. Further, the dies can mitigateeffects of thermal or mechanical gradients and associated impact onpiezoresistance, piezo-Hall, and thermoelectric effects, as well asreduce 1/frequency noise.

A processor used in the present sensor apparatus and method can beimplemented using software, firmware, hardware, or any appropriatecombination thereof, as known to one of skill in the art. These may besupplemented by, or incorporated in, specially-designedapplication-specific integrated circuits (ASICs) or field programmablegate arrays (FPGAs). The present method can be implemented by computerexecutable instructions, such as program modules, which are executed bythe processor. Generally, program modules include routines, objects,data components, data structures, algorithms, and the like.

Instructions for carrying out the various process tasks, calculations,and generation of other data used in the operation of the methodsdescribed herein can be implemented in software, firmware, or othercomputer or processor readable instructions. These instructions aretypically stored on any appropriate machine readable medium used forstorage of computer or processor readable instructions or datastructures.

Suitable processor readable media may include storage or memory mediasuch as magnetic or optical media. For example, storage or memory mediamay include volatile or non-volatile media such as Random Access Memory(RAM); Read Only Memory (ROM), Electrically Erasable Programmable ROM(EEPROM), flash memory, and the like; or any other media that can beused to carry or store desired program code in the form of computerexecutable instructions or data structures.

EXAMPLE EMBODIMENTS

Example 1 includes an ion sensor apparatus comprising at least one ionsensitive field effect transistor (ISFET) device configured to beexposed to a liquid; a reference electrode configured to contact theliquid to which the ISFET device is exposed; at least one magnetconfigured to intermittently expose the ISFET device to a magneticfield; and a processor operatively connected to the ISFET device and thereference electrode; wherein the processor modulates the magnetic fieldto produce a corresponding modulated output in resistance of the ISFETdevice, and modulation of a reported output value of the ion sensorapparatus.

Example 2 includes the sensor apparatus of Example 1, wherein the atleast one magnet comprises an electromagnet.

Example 3 includes the sensor apparatus of Example 2, wherein theelectromagnet is selectively activated by the processor tointermittently expose the ISFET device to the magnetic field produced bythe electromagnet.

Example 4 includes the sensor apparatus of Example 1, wherein the atleast one magnet comprises a permanent magnet.

Example 5 includes the sensor apparatus of Example 4, further comprisinga magnetic shield that is configured to intermittently expose the ISFETdevice to the magnetic field produced by the permanent magnet.

Example 6 includes the sensor apparatus of any of Examples 1-5, furthercomprising a magnetic field sensor adjacent to the magnet and configuredto measure the magnitude or state of the magnetic field.

Example 7 includes the sensor apparatus of any of Examples 1-6, furthercomprising a counter electrode configured to contact the liquid to whichthe ISFET device is exposed, wherein the processor is operativelyconnected to the counter electrode.

Example 8 includes the sensor apparatus of any of Examples 1-7, furthercomprising at least one temperature sensor and a pressure sensor,wherein the temperature sensor and the pressure sensor are configured tocontact the liquid to which the ISFET device is exposed.

Example 9 includes the sensor apparatus of any of Examples 1-8, furthercomprising a flow tube configured to receive the liquid, wherein theISFET device is disposed within the flow tube.

Example 10 includes the sensor apparatus of Example 9, furthercomprising a flow valve that controls the flow of the liquid into theflow tube.

Example 11 includes the sensor apparatus of any of Examples 1-10,wherein the ISFET device comprises a semiconductor substrate having atop surface; a source and a drain located in opposing side portions ofthe semiconductor substrate; a pair of electrical Hall contacts eachlocated in opposing side portions of the semiconductor substrate thatare adjacent to the side portions where the source and drain arelocated; and a gate channel, which is sensitive to ions in the liquid,located in a top central portion of the semiconductor substrate betweenthe source and the drain, and between the pair of electrical Hallcontacts; wherein the source, the drain, and the electrical Hallcontacts are separate from each other and located below the top surfaceof the semiconductor substrate.

Example 12 includes the sensor apparatus of any of Examples 1-11,wherein the reported output value is a pH measurement of the liquid.

Example 13 includes a method of compensating for ion sensor measurementerrors, the method comprising contacting a liquid with a portion of anISFET device; exposing the ISFET device to a modulated magnetic field toproduce a corresponding modulated output in resistance of the ISFETdevice; monitoring the modulated output to determine whether there is adifference between the modulated output and an expected output based onthe modulated magnetic field; determining an error compensation valuewhen there is a difference between the modulated output and the expectedoutput; and applying the error compensation value to null the differencebetween the modulated output and the expected output over a period oftime to maintain stability of ion sensor measurements.

Example 14 includes the method of Example 13, wherein the modulatedmagnetic field is from an electromagnet.

Example 15 includes the method of Example 14, wherein the electromagnetis selectively activated by a processor to expose the ISFET device tothe modulated magnetic field.

Example 16 includes the method of Example 13, wherein the modulatedmagnetic field is from at least one permanent magnet.

Example 17 includes the method of any of Examples 13-16, wherein theliquid comprises sea water, ground water, industrial chemicals, refinedchemicals, or crude oil.

Example 18 includes the method of any of Examples 13-17, wherein theISFET device is configured to measure a pH of the liquid.

Example 19 includes a semiconductor device comprising a semiconductorsubstrate having a top surface; a plurality of contacts located inopposing side portions of the semiconductor substrate; and an ionsensitive channel located in a top central portion of the semiconductorsubstrate between the contacts; wherein the contacts are separate fromeach other and located below the top surface of the semiconductorsubstrate.

Example 20 includes the device of Example 19, and further comprising oneor more rotary switches that selectively couple the contacts to one ormore voltage sources, such that the contacts are electrically configuredfor taking a pH measurement or a Hall effect potential measurement whenthe semiconductor device is employed in an ion sensor apparatus.

The present invention may be embodied in other specific forms withoutdeparting from its essential characteristics. The described embodimentsare to be considered in all respects only as illustrative and notrestrictive. The scope of the invention is therefore indicated by theappended claims rather than by the foregoing description. All changesthat come within the meaning and range of equivalency of the claims areto be embraced within their scope.

What is claimed is:
 1. An ion sensor apparatus, comprising at least oneion sensitive field effect transistor (ISFET) device configured to beexposed to a liquid; a reference electrode configured to contact theliquid to which the ISFET device is exposed; at least one magnetconfigured to intermittently expose the ISFET device to a magneticfield; and a processor operatively connected to the ISFET device and thereference electrode; wherein the processor modulates the magnetic fieldto produce a corresponding modulated output in resistance of the ISFETdevice, and modulation of a reported output value of the ion sensorapparatus.
 2. The sensor apparatus of claim 1, wherein the at least onemagnet comprises an electromagnet.
 3. The sensor apparatus of claim 2,wherein the electromagnet is selectively activated by the processor tointermittently expose the ISFET device to the magnetic field produced bythe electromagnet.
 4. The sensor apparatus of claim 1, wherein the atleast one magnet comprises a permanent magnet.
 5. The sensor apparatusof claim 4, further comprising a magnetic shield that is configured tointermittently expose the ISFET device to the magnetic field produced bythe permanent magnet.
 6. The sensor apparatus of claim 1, furthercomprising a magnetic field sensor adjacent to the magnet and configuredto measure the magnitude or state of the magnetic field.
 7. The sensorapparatus of claim 1, further comprising a counter electrode configuredto contact the liquid to which the ISFET device is exposed, wherein theprocessor is operatively connected to the counter electrode.
 8. Thesensor apparatus of claim 1, further comprising at least one temperaturesensor and a pressure sensor, wherein the temperature sensor and thepressure sensor are configured to contact the liquid to which the ISFETdevice is exposed.
 9. The sensor apparatus of claim 1, furthercomprising a flow tube configured to receive the liquid, wherein theISFET device is disposed within the flow tube.
 10. The sensor apparatusof claim 9, further comprising a flow valve that controls the flow ofthe liquid into the flow tube.
 11. The sensor apparatus of claim 1,wherein the ISFET device comprises: a semiconductor substrate having atop surface; a source and a drain located in opposing side portions ofthe semiconductor substrate; a pair of electrical Hall contacts eachlocated in opposing side portions of the semiconductor substrate thatare adjacent to the side portions where the source and drain arelocated; and a gate channel, which is sensitive to ions in the liquid,located in a top central portion of the semiconductor substrate betweenthe source and the drain, and between the pair of electrical Hallcontacts; wherein the source, the drain, and the electrical Hallcontacts are separate from each other and located below the top surfaceof the semiconductor substrate.
 12. The sensor apparatus of claim 1,wherein the reported output value is a pH measurement of the liquid. 13.A method of compensating for ion sensor measurement errors, the methodcomprising: contacting a liquid with a portion of an ion sensitive fieldeffect transistor (ISFET) device; exposing the ISFET device to amodulated magnetic field to produce a corresponding modulated output inresistance of the ISFET device; monitoring the modulated output todetermine whether there is a difference between the modulated output andan expected output based on the modulated magnetic field; determining anerror compensation value when there is a difference between themodulated output and the expected output; and applying the errorcompensation value to null the difference between the modulated outputand the expected output over a period of time to maintain stability ofion sensor measurements.
 14. The method of claim 13, wherein themodulated magnetic field is from an electromagnet.
 15. The method ofclaim 14, wherein the electromagnet is selectively activated by aprocessor to expose the ISFET device to the modulated magnetic field.16. The method of claim 13, wherein the modulated magnetic field is fromat least one permanent magnet.
 17. The method of claim 13, wherein theliquid comprises sea water, ground water, industrial chemicals, refinedchemicals, or crude oil.
 18. The method of claim 13, wherein the ISFETdevice is configured to measure a pH of the liquid.
 19. A semiconductordevice, comprising: a semiconductor substrate having a top surface; aplurality of contacts located in opposing side portions of thesemiconductor substrate; and an ion sensitive channel located in a topcentral portion of the semiconductor substrate between the contacts;wherein the contacts are separate from each other and located below thetop surface of the semiconductor substrate.
 20. The semiconductor deviceof claim 19, further comprising one or more rotary switches thatselectively couple the contacts to one or more voltage sources, suchthat the contacts are electrically configured for taking a pHmeasurement or a Hall effect potential measurement when thesemiconductor device is employed in an ion sensor apparatus.